A diode is a two-terminal electronic component that conducts current primarily in one direction
resistance; it has low in one direction, and high resistance in the other. A semiconductor diode, the
most common type today, is a crystalline piece of semiconductor material with a p–n junction
connected to two electrical terminals. Today, most diodes are made of silicon, but other materials such
as selenium and germanium are sometimes used.

The most common function of a diode is to allow an electric current to pass in one direction (called the
diode's forward direction), while blocking it in the opposite direction (the reverse direction). As such, the
diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called
rectification, and is used to convert alternating current (AC) to direct current (DC). Forms of rectifiers,
diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.

However, diodes can have more complicated behavior than this simple on–off action, because of their
nonlinear current-voltage characteristics. Semiconductor diodes begin conducting electricity only if a
certain threshold voltage or cut-in voltage is present in the forward direction (a state in which the diode
is said to be forward-biased). The voltage drop across a forward-biased diode varies only a little with the
current, and is a function of temperature; this effect can be used as a temperature sensor or as a voltage
reference.

A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor
materials and the doping impurities introduced into the materials during manufacture. These techniques
are used to create special-purpose diodes that perform many different functions. For example, diodes
are used to regulate voltage (Zener diodes), to protect circuits from high voltage surges (avalanche
diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio-frequency
oscillations (tunnel diodes, Gunn diodes, IMPATT diodes), and to produce light (light-emitting diodes).
Tunnel, Gunn and IMPATT diodes exhibit negative resistance, which is useful in microwave and
switching circuits.